Tectonic structures developed in oligocene limestones: Implications for New Zealand plate boundary deformation in North Canterbury

Abstract

Systematic analysis of brittle and semibrittle tectonic structures developed in two Oligocene limestones provides a good record of the post middle Cenozoic low‐strain deformational history in North Canterbury. These structures imply two periods of deformation. (1) Mild mid‐late Oligocene shortening along a NNE horizontal axis is inferred from fractures at a low angle to bedding. These are widespread throughout the northeastern South Island and indicate a regional tectonic event associated with the distal effects of major Oligocene extension in western New Zealand. (2) Late Pliocene ‐ Recent shortening was associated with the development of orthogonal fracture sets (many containing stylolites), macrofractures, small‐scale faults, and macroscopic folding. The late Cenozoic deformation is characterised by northwest‐southeast shortening, which is locally overprinted by east‐west and north‐south shortening associated with folding and the activation of macroscopic faults in basement. These late Pliocene and younger structures record a progressive increase in the magnitude of deformation and imply a rapid widening of the plate boundary zone during the late Pliocene ‐ early Pleistocene. This phenomenon, often associated with an increase in local uplift rates, is widespread throughout New Zealand and signifies a major change in the distribution and magnitude of strain accommodated across the plate boundary in the last 2 Ma. This may indicate an increase in the compressional component of relative plate motion across the plate boundary. Small‐scale structures imply a marked change of 60° in the azimuth of horizontal shortening between the mid‐late Oligocene and late Pliocene ‐ early Pleistocene. However, during the last 2–3 Ma, regional shortening in North Canterbury has not changed significantly in orientation. © 1992 Taylor & Francis Group, LLC.